1. Technical Field
The present invention relates to wireless networks, and, more particularly, to managing transmission power between base stations and mobile stations.
2. Description of Related Art
a. CDMA Networks Generally
Many people use mobile stations, such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These mobile stations and networks typically communicate with each other over a radio frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are often referred to as “1×RTT networks” (or “1× networks” for short), which stands for “Single Carrier Radio Transmission Technology.” These networks typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. When a mobile station is positioned in one of these coverage areas, it can communicate over the air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier, which may actually be a pair of frequencies, with the base station transmitting to the mobile station on one of the frequencies, and the mobile station transmitting to the base station on the other. This is known as frequency division duplex (FDD). The base-station-to-mobile-station link is known as the forward link, while the mobile-station-to-base-station link is known as the reverse link.
Furthermore, using a sector as an example of a wireless coverage area, base stations may provide service in a given sector on one carrier, or on more than one. An instance of a particular carrier in a particular sector is referred to herein as a sector/carrier. In a typical CDMA system, using a configuration known as radio configuration 3 (RC3), a base station can, on a given sector/carrier, transmit forward-link data on a maximum of 64 distinct channels at any given time, each channel corresponding to a unique 64-bit code known as a Walsh code. Of these channels, typically, 61 of them are available as traffic channels (for user data), while the other 3 are reserved for administrative channels known as the pilot, paging, and sync channels.
When a base station instructs a mobile station—operating on a particular sector/carrier—to use a particular traffic channel for a communication session, such as a voice call or a data session, the base station does so by instructing the mobile station to tune to a particular one of the 61 Walsh-coded traffic channels on that sector/carrier. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. And, in addition to that Walsh-coded forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station.
b. Forward-Link Transmission-Power Management                i. Forward-Link Frame Error Rate (FFER)        
In typical CDMA systems, communication between a base station and a mobile station involves (i) the base station sending data units known as frames to the mobile station on the forward link and (ii) the mobile station sending frames to the base station on the reverse link. These frames may carry administrative messages, voice data, SMS messages, packet data, and/or any other type of data. Focusing on the forward link, some of the frames received by the mobile station contain errors as a result of imperfect transfer, while some do not. Thus, a ratio can be computed between (i) the number of error-containing frames received by the mobile station over a given time period and (ii) the total number of frames received by the mobile station over that same time period. This ratio is known as the forward-link frame error rate (FFER). Note that the FFER calculations may also account for frames that are not received at all by the mobile station.
And other things being more or less equal, the more power the base station allocates for transmission to the mobile station on a traffic channel, the lower the mobile station's FFER will be. The mobile station periodically (e.g. once every 100 or 200 frames) computes its FFER, and reports it to the base station. The base station then adjusts its transmission power accordingly for that traffic channel. If the FFER is too high with respect to what is deemed to be an acceptable threshold, the base station increases transmission power in an effort to reduce the FFER. If the FFER is below the threshold, the base station may allocate less power to that mobile station, to have more available for other mobile stations. This back-and-forth calibration is conducted in an attempt to keep the mobile station's FFER at or just below the acceptable threshold, referred to herein as the “FFER target,” which may be configured to be in the neighborhood of 2%.
Note that different situations may present themselves on a sector/carrier at different times. For example, the number of mobile stations using traffic channels can vary between just a few, such as 5 or 10, to a large number, such as 25, 30, or more. Also, variables such as terrain, weather, buildings, other mobile stations, other types of interference, and a mobile station's distance from the base station can affect the FFER that a mobile station experiences and reports, and therefore, in turn, affect the amount of power that the base station allocates for transmitting forward-link data to the mobile station. Since base stations have only a finite total amount of power that they can allocate to the mobile stations on a sector/carrier, increasing the transmission power to some or all of those mobile stations (in order to keep their respective FFERs acceptably low) generally results in the base station being able to serve fewer mobile stations on that sector/carrier. That is, it reduces capacity on the sector/carrier.                ii. The Logarithmic Ratio Ec/Ior         
As explained, in typical CDMA systems, each base station can allocate a certain amount of power to transmitting on each sector/carrier on which it is providing service. The base station divides this power among any active traffic channels (over which the base station is transmitting data, such as voice and/or packet data, to mobile stations operating on that sector/carrier), as well as among the pilot, paging, and sync channels for the sector/carrier. Periodically, the base station calculates a ratio of (a) the power that the base station is allocating for transmitting the pilot channel on the sector/carrier (the “pilot-channel power level”) with (b) the power that the base station is allocating for transmitting all (i.e. pilot, paging, sync, and traffic) channels on the sector/carrier (the “all-channel power level”).
This ratio is a base-10 logarithmic one, and is known as “Ec/Ior.” The pilot-channel power level is referred to as “Ec”—“energy per chip.” The all-channel power level is referred to as “Ior”. Ec and Ior can each be expressed in Watts (W), milliwatts (mW), or any other suitable units of measure. Note that Ec and Ior are themselves often expressed as base-10 logarithmic ratios, with respect to a reference power level of 1 mW. In that case, Ec and Ior would each typically be expressed using the unit “dBm,” where “dB” indicates decibels and “m” indicates the reference power level. So, Ec can be expressed as the base-10 logarithmic ratio of the pilot-channel power level (in mW) and 1 mW. And Ior can be expressed as the base-10 logarithmic ratio of the all-channel power level (in mW) and 1 mW.
Ec/Ior is typically expressed as the base-10 logarithmic ratio of the pilot-channel power level and the all-channel power level, each of which may be measured in Watts. As such, the typical unit of measure for Ec/Ior is the decibel (dB). As an example, if a base station were allocating about 2 W (2000 mW) for the pilot channel, Ec would be about 33 dBm, calculated as 10*log((2000 mW)/(1 mW)). And if the base station were allocating a total of about 10 W (10,000 mW) for the pilot, paging, sync, and active traffic channels, Ior would be about 40 dBm, calculated as 10*log((10000 mW)/(1 mW)). In this example, Ec/Ior would be about −7 dB, calculated as 10*log((2 W)/(10 W)). Note that Ec/Ior will always be negative, as long as at least some power is allocated for any one or any combination of the paging, sync, and traffic channels.
As another example, a typical base station may have 16 W of power that it can potentially use for transmitting all channels on a sector/carrier, and may allocate 15% (2.4 W) of that for the pilot channel, 10% (1.6 W) for the paging channel, and 5% (0.8 W) for the sync channel. When that base station is not serving any mobile stations on active traffic channels on the sector/carrier, i.e. when the sector/carrier is “unloaded,” Ec/Ior would be approximately −3 dB, calculated as 10*log(2.4W/4.8W), which, then, would be about as high as Ec/Ior gets. Thus, for reference, anything close to −3 dB may be considered relatively high for Ec/Ior.
And when that same base station is at or near capacity (“fully loaded”), the 15% of its potential sector/carrier power that it is allocating for the pilot channel would shrink from being half of its power output on the sector/carrier (in the unloaded scenario) to, not surprisingly, being about 15% of its power output. This would yield an Ec/Ior of approximately −8 dB, calculated as 10*log(2.4W/16W), which, then would be about as low as Ec/Ior gets. Thus, for reference, anything close to −8 dB may be considered relatively low for Ec/Ior. In fact, a typical base station may stop accepting new mobile stations on a sector/carrier once Ec/Ior degrades to about −8 dB. Thus, Ec/Ior can impact sector/carrier capacity as well.
When Ec/Ior is relatively high, this could mean a number of things. For example, there could be only a few mobile stations on the sector/carrier, which would lead to a higher ratio of (i) pilot-channel power allocation to (ii) total power allocation (with relatively few traffic channels to which to allocate power). Instead or in addition, it could mean that the RF conditions are favorable, such that no (or relatively few) mobile stations are experiencing a poor FFER. In that situation, there would be no (or relatively few) mobile stations inducing the base station to increase power on the traffic channels. This would tend to keep the value of Ec/Ior relatively high. And other possibilities exist as well.
When Ec/Ior is relatively low, this also could mean a number of things. For example, there could be a relatively high number of mobile stations on the sector/carrier, and thus a high number of active traffic channels contributing to a high value of Ior, and thus a low value of Ec/Ior. Instead or in addition, it could mean that the RF conditions are poor (e.g., due to terrain, weather, interference, etc.); in that case, mobile stations would likely experience a poor FFER, and induce the base station to increase power on the traffic channels, which would contribute to a higher Ior and thus a lower Ec/Ior. And other possibilities exist as well.